When you convert an image from one of the color modes to the grayscale mode, Photoshop normally weights the values of each color channel in a way that retains the apparent brightness of the overall image. For example, when you convert an image from RGB, Photoshop weights red more heavily than blue when computing dark values. This is because red is a darker-looking color than blue (much as that might seem contrary to popular belief).
Tip If you choose Image Mode Grayscale while viewing a single color channel, though, Photoshop retains all brightness values in that channel only and abandons * the data in the other channels. This can be an especially useful technique for rescu ing a grayscale image from a bad RGB scan.
So before switching to the grayscale mode, be sure to look at the individual color channels — particularly the red and green channels (the blue channel frequently contains substandard detail) — to see how each channel might look on its own. To browse the channels, press Ctrl+1 for red, Ctrl+2 for green, and Ctrl+3 for blue. Or Ctrl+1 for cyan, Ctrl+2 for magenta, Ctrl+3 for yellow, and Ctrl+4 for black. Or even Ctrl+1 for luminosity, Ctrl+2 for a, and Ctrl+3 for b. Chapter 16 describes color channels in more detail.
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